Differential inhibition of cellular and Sindbis virus translation by brefeldin A

Screening of oxytetracycline-degrading strains in the intestine of the black soldier fly larvae and their degradation characteristics

The presence of excessive antibiotic residues poses a significant threat to human health and the environment. This study was designed to identify an effective oxytetracycline (OTC)-degrading strain through the screening of the intestine of black soldier fly larvae (BSFL). A strain designated "B2" was selected using a series of traditional microbial screening methods. It could be identified as Enterococcus faecalis by Gram staining and 16S rDNA sequencing, with a similarity of 99.93%. Its ability to degrade OTC was then assessed using high-performance liquid chromatography (HPLC). The degradation of the strain was characterized using a one-way test to assess the effects of the substrate concentration, inoculum amount, and initial pH on the degrading bacteria. The results indicate that strain B2 exhibited optimal OTC-degrading performance at a substrate concentration of 50 mg/L, with an inoculum amount of 6% and a pH value of 5.0. Specifically, strain B2 achieved degradation rates of 71.11%, 56.14%, and 45.03%. These findings demonstrate the effectiveness of strain B2 in degrading OTC, indicating its potential for use in environmental remediation efforts.

Canonical and Noncanonical Autophagy as Potential Targets for COVID-19

The SARS-CoV-2 pandemic necessitates a review of the molecular mechanisms underlying cellular infection by coronaviruses, in order to identify potential therapeutic targets against the associated new disease (COVID-19). Previous studies on its counterparts prove a complex and concomitant interaction between coronaviruses and autophagy. The precise manipulation of this pathway allows these viruses to exploit the autophagy molecular machinery while avoiding its protective apoptotic drift and cellular innate immune responses. In turn, the maneuverability margins of such hijacking appear to be so narrow that the modulation of the autophagy, regardless of whether using inducers or inhibitors (many of which are FDA-approved for the treatment of other diseases), is usually detrimental to viral replication, including SARS-CoV-2. Recent discoveries indicate that these interactions stretch into the still poorly explored noncanonical autophagy pathway, which might play a substantial role in coronavirus replication. Still, some potential therapeutic targets within this pathway, such as RAB9 and its interacting proteins, look promising considering current knowledge. Thus, the combinatory treatment of COVID-19 with drugs affecting both canonical and noncanonical autophagy pathways may be a turning point in the fight against this and other viral infections, which may also imply beneficial prospects of long-term protection.

Role of the Guanine Nucleotide Exchange Factor GBF1 in the Replication of RNA Viruses

The guanine nucleotide exchange factor GBF1 is a well-known factor that can activate different ADP-ribosylation factor (Arf) proteins during the regulation of different cellular vesicular transport processes. In the last decade, it has become increasingly evident that GBF1 can also regulate different steps of the replication cycle of RNA viruses belonging to different virus families. GBF1 has been shown not only to facilitate the intracellular traffic of different viral and cellular elements during infection, but also to modulate the replication of viral RNA, the formation and maturation of viral replication complexes, and the processing of viral proteins through mechanisms that do not depend on its canonical role in intracellular transport. Here, we review the various roles that GBF1 plays during the replication of different RNA viruses.

A compendium of small molecule direct-acting and host-targeting inhibitors as therapies against alphaviruses

Alphaviruses were amongst the first arboviruses to be isolated, characterized and assigned a taxonomic status. They are globally widespread, infecting a large variety of terrestrial animals, birds, insects and even fish. Moreover, they are capable of surviving and circulating in both sylvatic and urban environments, causing considerable human morbidity and mortality. The re-emergence of Chikungunya virus (CHIKV) in almost every part of the world has caused alarm to many health agencies throughout the world. The mosquito vector for this virus, Aedes, is globally distributed in tropical and temperate regions and capable of thriving in both rural and urban landscapes, giving the opportunity for CHIKV to continue expanding into new geographical regions. Despite the importance of alphaviruses as human pathogens, there is currently no targeted antiviral treatment available for alphavirus infection. This mini-review discusses some of the major features in the replication cycle of alphaviruses, highlighting the key viral targets and host components that participate in alphavirus replication and the molecular functions that were used in drug design. Together with describing the importance of these targets, we review the various direct-acting and host-targeting inhibitors, specifically small molecules that have been discovered and developed as potential therapeutics as well as their reported in vitro and in vivo efficacies.

Cellular unfolded protein response against viruses used in gene therapy

Viruses are excellent vehicles for gene therapy due to their natural ability to infect and deliver the cargo to specific tissues with high efficiency. Although such vectors are usually "gutted" and are replication defective, they are subjected to clearance by the host cells by immune recognition and destruction. Unfolded protein response (UPR) is a naturally evolved cyto-protective signaling pathway which is triggered due to endoplasmic reticulum (ER) stress caused by accumulation of unfolded/misfolded proteins in its lumen. The UPR signaling consists of three signaling pathways, namely PKR-like ER kinase, activating transcription factor 6, and inositol-requiring protein-1. Once activated, UPR triggers the production of ER molecular chaperones and stress response proteins to help reduce the protein load within the ER. This occurs by degradation of the misfolded proteins and ensues in the arrest of protein translation machinery. If the burden of protein load in ER is beyond its processing capacity, UPR can activate pro-apoptotic pathways or autophagy leading to cell death. Viruses are naturally evolved in hijacking the host cellular translation machinery to generate a large amount of proteins. This phenomenon disrupts ER homeostasis and leads to ER stress. Alternatively, in the case of gutted vectors used in gene therapy, the excess load of recombinant vectors administered and encountered by the cell can trigger UPR. Thus, in the context of gene therapy, UPR becomes a major roadblock that can potentially trigger inflammatory responses against the vectors and reduce the efficiency of gene transfer.

Virus versus host cell translation love and hate stories

Regulation of protein synthesis by viruses occurs at all levels of translation. Even prior to protein synthesis itself, the accessibility of the various open reading frames contained in the viral genome is precisely controlled. Eukaryotic viruses resort to a vast array of strategies to divert the translation machinery in their favor, in particular, at initiation of translation. These strategies are not only designed to circumvent strategies common to cell protein synthesis in eukaryotes, but as revealed more recently, they also aim at modifying or damaging cell factors, the virus having the capacity to multiply in the absence of these factors. In addition to unraveling mechanisms that may constitute new targets in view of controlling virus diseases, viruses constitute incomparably useful tools to gain in-depth knowledge on a multitude of cell pathways.

Large Arf1 guanine nucleotide exchange factors: evolution, domain structure, and roles in membrane trafficking and human disease

The Sec7 domain ADP-ribosylation factor (Arf) guanine nucleotide exchange factors (GEFs) are found in all eukaryotes, and are involved in membrane remodeling processes throughout the cell. This review is focused on members of the GBF/Gea and BIG/Sec7 subfamilies of Arf GEFs, all of which use the class I Arf proteins (Arf1-3) as substrates, and play a fundamental role in trafficking in the endoplasmic reticulum (ER)—Golgi and endosomal membrane systems. Members of the GBF/Gea and BIG/Sec7 subfamilies are large proteins on the order of 200 kDa, and they possess multiple homology domains. Phylogenetic analyses indicate that both of these subfamilies of Arf GEFs have members in at least five out of the six eukaryotic supergroups, and hence were likely present very early in eukaryotic evolution. The homology domains of the large Arf1 GEFs play important functional roles, and are involved in interactions with numerous protein partners. The large Arf1 GEFs have been implicated in several human diseases. They are crucial host factors for the replication of several viral pathogens, including poliovirus, coxsackievirus, mouse hepatitis coronavirus, and hepatitis C virus. Mutations in the BIG2 Arf1 GEF have been linked to autosomal recessive periventricular heterotopia, a disorder of neuronal migration that leads to severe malformation of the cerebral cortex. Understanding the roles of the Arf1 GEFs in membrane dynamics is crucial to a full understanding of trafficking in the secretory and endosomal pathways, which in turn will provide essential insights into human diseases that arise from misregulation of these pathways.

A critical role of a cellular membrane traffic protein in poliovirus RNA replication

Replication of many RNA viruses is accompanied by extensive remodeling of intracellular membranes. In poliovirus-infected cells, ER and Golgi stacks disappear, while new clusters of vesicle-like structures form sites for viral RNA synthesis. Virus replication is inhibited by brefeldin A (BFA), implicating some components(s) of the cellular secretory pathway in virus growth. Formation of characteristic vesicles induced by expression of viral proteins was not inhibited by BFA, but they were functionally deficient. GBF1, a guanine nucleotide exchange factor for the small cellular GTPases, Arf, is responsible for the sensitivity of virus infection to BFA, and is required for virus replication. Knockdown of GBF1 expression inhibited virus replication, which was rescued by catalytically active protein with an intact N-terminal sequence. We identified a mutation in GBF1 that allows growth of poliovirus in the presence of BFA. Interaction between GBF1 and viral protein 3A determined the outcome of infection in the presence of BFA.

Mouse hepatitis coronavirus RNA replication depends on GBF1-mediated ARF1 activation

Coronaviruses induce in infected cells the formation of double membrane vesicles, which are the sites of RNA replication. Not much is known about the formation of these vesicles, although recent observations indicate an important role for the endoplasmic reticulum in the formation of the mouse hepatitis coronavirus (MHV) replication complexes (RCs). We now show that MHV replication is sensitive to brefeldin A (BFA). Consistently, expression of a dominant-negative mutant of ARF1, known to mimic the action of the drug, inhibited MHV infection profoundly. Immunofluorescence analysis and quantitative electron microscopy demonstrated that BFA did not block the formation of RCs per se, but rather reduced their number. MHV RNA replication was not sensitive to BFA in MDCK cells, which are known to express the BFA-resistant guanine nucleotide exchange factor GBF1. Accordingly, individual knockdown of the Golgi-resident targets of BFA by transfection of small interfering RNAs (siRNAs) showed that GBF1, but not BIG1 or BIG2, was critically involved in MHV RNA replication. ARF1, the cellular effector of GBF1, also appeared to be involved in MHV replication, as siRNAs targeting this small GTPase inhibited MHV infection significantly. Collectively, our results demonstrate that GBF1-mediated ARF1 activation is required for efficient MHV RNA replication and reveal that the early secretory pathway and MHV replication complex formation are closely connected.

Coronaviruses are the causative agents of many respiratory and enteric infections in humans and animals. As with all viruses, virtually all of the steps of their infection cycle depend on host cellular factors. As the first and most crucial step after their entry into cells, coronaviruses assemble their replication complexes (RCs) in association with characteristic, newly induced membranous structures. The cellular pathways hijacked by these plus-strand RNA viruses to create these “factories” have not been elucidated. Here, we study the involvement of the secretory pathway in mouse hepatitis coronavirus (MHV) replication by using the drug brefeldin A (BFA), which is known to interfere with ER–Golgi membrane traffic by inhibiting the activation of ADP-ribosylation factor (ARF) small GTPases. Our observations show that MHV RNA replication is sensitive to BFA. In agreement herewith we demonstrate, by using various techniques, that the BFA-sensitive guanidine nucleotide exchange factor GBF1 and its downstream effector ARF1 are of critical importance for coronavirus replication. From our results we conclude that MHV RNA replication depends on GBF1-mediated ARF1 activation. Our study provides new insights into the close connection between MHV replication and the early secretory pathway.

Different effect of proteasome inhibition on vesicular stomatitis virus and poliovirus replication

Proteasome activity is an important part of viral replication. In this study, we examined the effect of proteasome inhibitors on the replication of vesicular stomatitis virus (VSV) and poliovirus. We found that the proteasome inhibitors significantly suppressed VSV protein synthesis, virus accumulation, and protected infected cells from toxic effect of VSV replication. In contrast, poliovirus replication was delayed, but not diminished in the presence of the proteasome inhibitors MG132 and Bortezomib. We also found that inhibition of proteasomes stimulated stress-related processes, such as accumulation of chaperone hsp70, phosphorylation of eIF2α, and overall inhibition of translation. VSV replication was sensitive to this stress with significant decline in replication process. Poliovirus growth was less sensitive with only delay in replication. Inhibition of proteasome activity suppressed cellular and VSV protein synthesis, but did not reduce poliovirus protein synthesis. Protein kinase GCN2 supported the ability of proteasome inhibitors to attenuate general translation and to suppress VSV replication. We propose that different mechanisms of translational initiation by VSV and poliovirus determine their sensitivity to stress induced by the inhibition of proteasomes. To our knowledge, this is the first study that connects the effect of stress induced by proteasome inhibition with the efficiency of viral infection.